In an automatic transmission (AT) and a continuously variable transmission (CVT) for automobiles, in order to hold a hydraulic pressure, a seal ring which seals an annular gap between a shaft and a housing which rotate relative to each other is provided. With reference to FIGS. 33 and 34, a seal ring according to a conventional art will be described. FIG. 33 is a schematic cross-sectional view showing a seal ring according to the conventional art in a condition in which the hydraulic pressure is not held. FIG. 34 is a schematic cross-sectional view showing the seal ring according to the conventional art in a condition in which the hydraulic pressure is held. In the case of a seal ring 600 according to the conventional art, the seal ring 600 is configured so as to seal an annular gap between a shaft 400 and a shaft hole of a housing 500 by being attached to an annular groove 410 provided on an outer periphery of the shaft 400, and being slidably in contact with an inner peripheral surface of the shaft hole of the housing 500, through which the shaft 400 is inserted, and a side wall surface of the annular groove 410.
In the seal ring 600 which is used for the above described purpose, it is required to sufficiently reduce a sliding torque. Accordingly, the circumference of an outer peripheral surface of the seal ring 600 is configured to be shorter than the circumference of the inner peripheral surface of the shaft hole of the housing 500, thus it is configured to have no interference. Consequently, in a condition in which an engine of an automobile is operated and the hydraulic pressure is high, the seal ring 600 is expanded in diameter by the hydraulic pressure and makes close contact with the inner peripheral surface of the shaft hole and the side wall surface of the annular groove 410 to thereby exhibit a function of sufficiently holding the hydraulic pressure (see FIG. 34). In contrast to this, in a condition in which the hydraulic pressure is not applied due to stoppage of the engine, the seal ring 600 is configured so as to be apart from the inner peripheral surface of the shaft hole and the side wall surface of the annular groove 410 (see FIG. 33).
However, in the case of the seal ring 600 which is configured in the above described manner, the seal ring 600 does not exhibit a sealing function in the condition in which the hydraulic pressure is not applied. Accordingly, in a configuration such as AT or CVT, in which gear shifting control is performed by oil which is pressure-fed by a hydraulic pump, when it is in a no-load condition in which the hydraulic pump is stopped (e.g., during an idling stop), the oil sealed by the seal ring 600 may no longer be sealed and return to an oil pan, thereby leaving no oil in the vicinity of the seal ring 600. Consequently, when the engine is started (re-started) from such a condition, the operation is started in a condition in which no oil is present in the vicinity of the seal ring 600; hence, without lubrication, a problem arises in that responsiveness or operativity may be poor.